Ventricular fibrillation is a life-threatening cardiac arrhythmia which, if untreated, will most probably cause death of the patient in whom it occurs. There exists an identifiable population of patients who have a high risk of ventricular fibrillation and for whom no effective medical therapy is currently available. An alternative therapy is implantation of an automatic defibrillator. Most of the automatic implantable defibrillators which have been proposed and are under development rely on a transvenous monopolar or bipolar catheter to accomplish ventricular defibrillation. The defibrillating electrode system is a critical component of the automatic implantable defibrillator, and recent data suggest that an inappropriately designed electrode system will severely limit the efficacy of the implantable device. It seems that there is a critical level of current density which, if exceeded, produces a variety of highly undesirable side effects. Inadequate experimental data and the absence of a model which describes the influence of electrode variables on current-carrying ability of the electrode system prohibit design of suitable electrode systems. The purpose of this investigation is to develop a model for implantable defibrillating electrodes and to utilize the predictions from the model in the implementation of an optimal implantable defibrillating electrode system. Electrodes designed on the basis of results from the model studies will be constructed and tested in adult, human-size animal subjects.